1. Field of the Invention
This invention relates to new polyisocyanate mixtures which contain both carboxyl groups and aliphatically and/or cycloaliphatically bound uretdione groups, to processes for the production of these polyisocyanate mixtures and to their use as binders for coating compositions or as reactants for isocyanate-reactive compounds in the production of high molecular weight plastics. 2. Description of the Prior Art
Polyisocyanates which contain uretdione groups and are free from masking agents are heat-activatable, crosslinking agents for intermediate products containing isocyanate-reactive groups for the production of plastics. These polyisocyanates are interesting because they are capable of reacting with compounds containing isocyanate-reactive groups both at room temperature (free NCO groups) and at relatively high temperatures (blocked NCO groups) in an addition reaction. Thus, allophanate groups are formed, for example, from hydroxyl groups and uretdione groups at elevated temperatures. Of industrial significance are inter alia compounds containing cycloaliphatically bound uretdione groups (DE-AS 3,030,513) or aliphatically bound uretdione groups (DE-OS 3,437,635).
Compounds containing isocyanate groups and free carboxyl groups are also known as intermediates for the production of plastics In the synthesis of aqueous polyurethane dispersions for example, NCO prepolymers having chemically incorporated carboxyl groups can be produced as intermediates through the use of sterically hindered hydroxyalkane carboxylic acids, such as dimethylol propionic acid, as disclosed in U.S. Pat. No. 3,412,054.
It has now surprisingly been found that the combination of these two principles, i.e., the simultaneous incorporation of (cyclo)aliphatically bound uretdione groups and carboxyl groups in modified polyisocyanate mixtures, results in a number of remarkable properties. Despite the simultaneous presence of isocyanate groups and carboxyl groups, the polyisocyanate mixtures according to the invention described in detail hereinafter remain stable in storage for at least three months at room temperature, are soluble or dispersible in water after neutralization with tertiary amines, crosslink spontaneously at elevated temperatures, demonstrate the property of being film-forming even in the absence of other auxiliaries and additives and are useful in various reactions with corresponding reactants by virtue of the simultaneous presence of three different reactive groups. In particular, they are suitable in combination with compounds containing isocyanate-reactive groups for the production of high molecular weight plastics.